Phone seems to be jailbroken (I can get to Cydia and install things over Wifi there).
Once I plug in my T-Mobile SIM card, it does not recognize the carrier as T-Mobile.
Phone appears to not be unlocked.

Any help with what I need to do to unlock this phone would be greatly appreciated.
Thank you very much.

Already read it. . the thread is dated from november 2010 and is very old. I heard somewhere you can downgrade from 5.15.04 baseband to a lower version. if I don't have to go to 6.15 then I prefer not to. so my post last to find out other ways of doing that unlock without having to upgrade to baseband.
I guess is the next question is do I have to go to 6.15 or are there other ways of doing this, where I don't have to upgrade.
Thank you for your help.

Wrong, you didn't read seriously, if you had you 'd have seen at the bottom of the post that it was updated only a month ago, moreover there's info on the only exception in the general rule that YOU CAN'T DOWNGRADE BASEBAND.
So let me repeat: read that stickie and check if your bootloader is a one in a million to allow downgrade, then proceed to install ipad bb or no unlock.
Good luck!

You can't downgrade the baseband. Your bootloader is 6.04. The only device that can downgrade basebands is one with bootloader 5.08. Hope that has answered your question. Your only choice is to upgrade to the iPad bb, which may break GPS.

Edit: those other numerous places you speak of, I wouldn't trust those guides if they don't confirm that your bootloader must be 5.08. If they are claiming that other devices without that bootloader can downgrade basebands, they're incorrect. You could always try those guides that you are seeing though, I'm just saying they won't work on your particular device.

..there are various places who give instructions for downgrading back from 6.15, so that is why I am asking. Thank you for your reply. Not very personable but nevertheless.

If you use those various guides on a phone that's not a 3G with bootloader 5.08 then I guarantee that you'll be on here later asking why there's no imei number and bluetooth and wifi are greyed out. The answer to that will be that the baseband chip has been corrupted and you now have a fancy iPod classic. iAnnie is right and a very good person to listen to she knows what she's talking about.